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                                <h1 id="prerequisite-reducer-concepts">Prerequisite Reducer Concepts</h1>
<p>As described in <a href="../../basics/Reducers.html">Reducers</a>, a Redux <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> function:</p>
<ul>
<li>Should have a signature of <code>(previousState, action) =&gt; newState</code>, similar to the type of function you would pass to <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce" target="_blank"><code>Array.prototype.reduce(reducer, ?initialValue)</code></a></li>
<li>Should be &quot;pure&quot;, which means the <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a>:<ul>
<li>Does not <em>perform side effects</em> (such as calling API&apos;s or modifying non-local objects or variables).</li>
<li>Does not <em>call non-pure functions</em> (like <code>Date.now</code> or <code>Math.random</code>).</li>
<li>Does not <em>mutate</em> its arguments. If the <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> updates <a href="../../Glossary.html#state" class="glossary-term" title="">state</a>, it should not <em>modify</em> the <strong>existing</strong> <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> object in-place.  Instead, it should generate a <strong>new</strong> object containing the necessary changes. The same approach should be used for any sub-objects within <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> that the <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> updates.</li>
</ul>
</li>
</ul>
<blockquote>
<h5 id="note-on-immutability-side-effects-and-mutation">Note on immutability, side effects, and mutation</h5>
<p>Mutation is discouraged because it generally breaks time-travel debugging, and React Redux&apos;s <code>connect</code> function:</p>
<ul>
<li>For time traveling, the Redux DevTools expect that replaying recorded actions would output a <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> value, but not change anything else. <strong>Side effects like mutation or asynchronous behavior will cause time travel to alter behavior between steps, breaking the application</strong>.</li>
<li>For React Redux, <code>connect</code> checks to see if the props returned from a <code>mapStateToProps</code> function have changed in order to determine if a component needs to update.  To improve performance, <code>connect</code> takes some shortcuts that rely on the <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> being immutable, and uses shallow reference equality checks to detect changes. This means that <strong>changes made to objects and arrays by direct mutation will not be detected, and components will not re-render</strong>.</li>
</ul>
<p>Other side effects like generating unique IDs or timestamps in a <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> also make the code unpredictable and harder to debug and test.</p>
</blockquote>
<p>Because of these rules, it&apos;s important that the following core concepts are fully understood before moving on to other specific techniques for organizing Redux reducers:</p>
<h4 id="redux-reducer-basics">Redux Reducer Basics</h4>
<p><strong>Key concepts</strong>:</p>
<ul>
<li>Thinking in terms of <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> and <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> shape</li>
<li>Delegating update responsibility by slice of <a href="../../Glossary.html#state" class="glossary-term" title="">state</a> (<em><a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> composition</em>)</li>
<li>Higher order reducers</li>
<li>Defining <a href="../../Glossary.html#reducer" class="glossary-term" title="">reducer</a> initial <a href="../../Glossary.html#state" class="glossary-term" title="">state</a></li>
</ul>
<p><strong>Reading list</strong>:</p>
<ul>
<li><a href="../../basics/Reducers.html">Redux Docs: Reducers</a></li>
<li><a href="../ReducingBoilerplate.html">Redux Docs: Reducing Boilerplate</a></li>
<li><a href="../ImplementingUndoHistory.html">Redux Docs: Implementing Undo History</a></li>
<li><a href="../../api/combineReducers.html">Redux Docs: <code>combineReducers</code></a></li>
<li><a href="http://slides.com/omnidan/hor#/" target="_blank">The Power of Higher-Order Reducers</a></li>
<li><a href="http://stackoverflow.com/questions/33749759/read-stores-initial-state-in-redux-reducer" target="_blank">Stack Overflow: Store initial state and <code>combineReducers</code></a></li>
<li><a href="http://stackoverflow.com/questions/35667775/state-in-redux-react-app-has-a-property-with-the-name-of-the-reducer" target="_blank">Stack Overflow: State key names and <code>combineReducers</code></a></li>
</ul>
<h4 id="pure-functions-and-side-effects">Pure Functions and Side Effects</h4>
<p><strong>Key Concepts</strong>:</p>
<ul>
<li>Side effects</li>
<li>Pure functions</li>
<li>How to think in terms of combining functions</li>
</ul>
<p><strong>Reading List</strong>:</p>
<ul>
<li><a href="http://jaysoo.ca/2016/01/13/functional-programming-little-ideas/" target="_blank">The Little Idea of Functional Programming</a></li>
<li><a href="http://web24studios.com/2015/10/understanding-programmatic-side-effects/" target="_blank">Understanding Programmatic Side Effects</a></li>
<li><a href="https://youtu.be/e-5obm1G_FY" target="_blank">Learning Functional Programming in Javascript</a></li>
<li><a href="https://www.sitepoint.com/an-introduction-to-reasonably-pure-functional-programming/" target="_blank">An Introduction to Reasonably Pure Functional Programming</a></li>
</ul>
<h4 id="immutable-data-management">Immutable Data Management</h4>
<p><strong>Key Concepts</strong>:</p>
<ul>
<li>Mutability vs immutability</li>
<li>Immutably updating objects and arrays safely</li>
<li>Avoiding functions and statements that mutate <a href="../../Glossary.html#state" class="glossary-term" title="">state</a></li>
</ul>
<p><strong>Reading List</strong>:</p>
<ul>
<li><a href="http://reactkungfu.com/2015/08/pros-and-cons-of-using-immutability-with-react-js/" target="_blank">Pros and Cons of Using Immutability With React</a></li>
<li><a href="http://t4d.io/javascript-and-immutability/" target="_blank">Javascript and Immutability</a></li>
<li><a href="http://wecodetheweb.com/2016/02/12/immutable-javascript-using-es6-and-beyond/" target="_blank">Immutable Data using ES6 and Beyond</a></li>
<li><a href="https://ryanfunduk.com/articles/immutable-data-from-scratch/" target="_blank">Immutable Data from Scratch</a></li>
<li><a href="../UsingObjectSpreadOperator.html">Redux Docs: Using the Object Spread Operator</a></li>
</ul>
<h4 id="normalizing-data">Normalizing Data</h4>
<p><strong>Key Concepts</strong>:</p>
<ul>
<li>Database structure and organization</li>
<li>Splitting relational/nested data up into separate tables</li>
<li>Storing a single definition for a given item</li>
<li>Referring to items by IDs</li>
<li>Using objects keyed by item IDs as lookup tables, and arrays of IDs to track ordering</li>
<li>Associating items in relationships</li>
</ul>
<p><strong>Reading List</strong>:</p>
<ul>
<li><a href="http://www.essentialsql.com/get-ready-to-learn-sql-database-normalization-explained-in-simple-english/" target="_blank">Database Normalization in Simple English</a></li>
<li><a href="https://egghead.io/lessons/javascript-redux-normalizing-the-state-shape" target="_blank">Idiomatic Redux: Normalizing the State Shape</a></li>
<li><a href="https://github.com/paularmstrong/normalizr" target="_blank">Normalizr Documentation</a></li>
<li><a href="https://tonyhb.gitbooks.io/redux-without-profanity/content/normalizer.html" target="_blank">Redux Without Profanity: Normalizr</a></li>
<li><a href="https://medium.com/@adamrackis/querying-a-redux-store-37db8c7f3b0f" target="_blank">Querying a Redux Store</a></li>
<li><a href="https://en.wikipedia.org/wiki/Associative_entity" target="_blank">Wikipedia: Associative Entity</a></li>
<li><a href="http://www.tomjewett.com/dbdesign/dbdesign.php?page=manymany.php" target="_blank">Database Design: Many-to-Many</a></li>
<li><a href="https://medium.com/@talkol/avoiding-accidental-complexity-when-structuring-your-app-state-6e6d22ad5e2a" target="_blank">Avoiding Accidental Complexity When Structuring Your App State</a></li>
</ul>

                                
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